740 



PRINCIPLES OF STRATIGRAPHY 



by year, and in so growing the latest deposits, whether derived from 

 the mountain or whether obtained through the reworking of the 

 previously deposited portion, will as a rule extend further out into 

 the plain than did those of previous periods of deposition. In other 

 words, each later formation will overlap the previous ones by a 

 margin commensurate with the increase in the size of the fan, and 

 beyond the margin of the previously formed bed it will come to rest 

 directly upon the floor of the pl<iin. This, overlapping of later 

 formed over earlier ones wull of course be a progressive one if the 

 growth is continuous. The essential point of difiference between this 

 type of overlap and that formed in a transgressing sea is that in 

 the subaerial fan the formations will primarily overlap one another 

 away from the source of supply of the jnaterial, while in the ma- 

 rine progressive overlap the overlap is toxvard the source of supply 

 of the material. The following diagrams will illustrate this differ- 



FiG. 153. Diagrams illustrating the relationship of the strata in nonmarine 

 progressive overlap (a) and marine progressive overlap (b). 

 The source of the material in each case is on the left. 



ence, the source of supply in each case being on the left. (Figs. 

 153, a, b.) It should, however, be noted here (Barrell-i) that at 

 the head of the delta an overlap toward the source of supply may 

 occur, since with the aggradation of the delta plain and the conse- 

 quent lowering of the river grade deposition may commence farther 

 upstream. Such effects are seen in interior basins, where the upper 

 beds extend farther toward the mountains from which the material 

 has been derived. It appears to be shown also in the case of the 

 Newark formation, where the upper beds extend farther north, 

 overlapping the lower ones. (Kummel-5 :^(?.) It may, however, 

 be questioned whether headward overlapping of the formations is 

 ever of marked character, since it is necessarily confined to the 

 stream channels supplying the detritus and probably does not spread 

 greatly in a lateral direction. In any case it cannot be compared in 

 extent and importance with the overlap on the margin of the fan, 

 which is always away from the source of supply. 



The coarsest material of the subaerial fan will of course be de- 

 posited near the head of the delta. Finer material may be carried 



